Since downsizing and weight saving of electronic devices have been advancing, secondary batteries whose energy density is high have been desired for their power source. A secondary battery is one that takes out chemical energy, which the positive-electrode active material and negative-electrode material possess, as electric energy by means of chemical reaction through electrolyte. In such secondary batteries, lithium-ion secondary batteries are secondary batteries, which possess a higher energy density, among those that have been put in practical use. Even among those, the spreading of organic-electrolyte-system lithium-ion secondary batteries (hereinafter being recited simply as “lithium-ion secondary batteries”) has been progressing.
For lithium-ion secondary battery, lithium-containing metallic composite oxides, such as lithium-cobalt composite oxides, have been used mainly as an active material for the positive electrode; and carbonaceous materials, which have a multi-layered structure that enables the insertion of lithium ions between the layers (i.e., the formation of lithium intercalation complex) and the discharge of lithium ions out from between the layers, have been used mainly as an active material for the negative electrode. The positive-electrode and negative-electrode polar plates are made in the following manner: these active materials, and a binder resin are dispersed in a solvent to make a slurry, respectively; then the resulting slurries are applied onto opposite faces of a metallic foil, namely, an electricity collector, respectively; and then the solvent is dry removed to form mixture-agent layers; and thereafter the resulting mixture-agent layers and electricity collector are compression molded with a roller pressing machine.
In the other secondary batteries as well, although the types of respective active materials, electricity collectors, and the like, differ, such secondary batteries have been available as those in which the active materials are bound or immobilized to the electricity collector by means of a binder resin similarly.
As for the binder resin on this occasion, polyvinylidene fluoride (hereinafter being abbreviated to as “PVdf”) has been used often for both of the electrodes. Since this binder resin is a fluorinated resin, the adhesiveness to electricity collectors is poor, and accordingly it is probable that the falling down of active materials might occur.
Moreover, as the negative-electrode active material for lithium secondary battery, the development of next-generation negative-electrode active materials, which possess a charge/discharge capacity that exceeds the theoretical capacity of carbonaceous material, has been advanced recently. For example, materials that include a metal, such as Si or Sn, which is capable of alloying with lithium, are regarded prospective. In the case of using Si or Sn, and so forth, for an active material, it is difficult to maintain the bonded state to electricity collector satisfactorily even when the aforementioned fluorinated resin is used for the binder, because the volumetric change of the aforementioned active material that is accompanied by the occlusion/release of Li at the time of charging/discharging is great. These materials exhibit a large rate of volumetric change that is accompanied by the insertion and elimination of lithium; and accordingly they are associated with such a drawback that the cyclic degradation is great considerably, because they are expanded and contracted repeatedly so that their active-material particles have been pulverized finely or have come to be detached.
In Patent Literature No. 1, there is a recitation on a negative electrode for secondary battery that has excellent cyclic performance, and in which the battery reliability at high temperatures is improved by means of binding the following together with a binder, such as polyimide or polyamide-imide, which has been known as a heat-resistant polymer: an active material containing an element that is capable of alloying with lithium; a catalytic element for promoting the growth of carbon nano-fibers; and composite particles containing carbon nano-fibers that have been grown from the active material's surface.
Moreover, in Patent Literature No. 2, a binder resinous composition for battery is disclosed, binder resinous composition in which a block copolymer is used, block copolymer in which nonpolar molecular species that do not have any ring on the principal-chain framework, and polar molecular species that have a ring on the principal-chain framework are bonded to each other. In the examples, it indicates that the cyclic life of nonaqueous-electrolytic-solution secondary batteries, which were made by using the binder resinous composition that included the block copolymer, was improved.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2006-339,092; and
Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-221,014.